This invention relates generally to composite porous membranes, and more particularly to, composite porous membranes compatible with ion exchange materials.
It is known that a porous membrane may have at least one property that is limited by the material from which the membrane is made. For example, a porous membrane made from expanded polytetrafluoroethylene (ePTFE) material has excellent hydrophobic properties, has low surface energy, and is extremely inert. Therefore, most materials are difficult to chemically or mechanically attach or connect to ePTFE.
U.S. Pat. No. 5,976,380 discloses a hydrophilic coating on a surface of a porous polytetrafluoroethylene (PTFE) membrane by wetting the membrane with the solution, applying the solution under pressure or pre-wetting the membrane with a wetting agent such as methanol, ethanol, isopropanal or acetone. U.S. Pat. Nos. 6,228,447; 6,410,084 6,676,993, and 6,854,603 disclose an improved membrane structure that is air permeable and treated to protect the ePTFE membrane from oil contamination by treating the surfaces and pores of the membrane. U.S. Pat. No. 6,270,844 discloses a method of impregnating a porous polymer substrate with supercritical fluid.
U.S. Pat. Nos. 5,547,551; 5,599,614 and 5,635,041 disclose a composite membrane for fuel cell application. The composite membrane includes an ePTFE membrane with its pores filled with an ion exchange material. Such a membrane can suffer from development of “pin holes” when ion exchange material separates from the surfaces defining the pores in the membrane. Pin holes have a detrimental effect on the performance of the composite membrane.
U.S. Pat. No. 4,469,744 discloses protective clothing containing a layer of highly fluorinated ion exchange polymer. A microporous polytetrafluoroethylene (PTFE) film has a solution of ion exchange material applied with the use of a vacuum. U.S. Pat. No. 4,902,308 discloses a film of porous expanded PTFE with exterior and interior surfaces coated with a metal salt of perfluoro-cation exchange polymer. U.S. Pat. No. 4,954,388 discloses a multi-layer composite membrane having a film of continuous ion exchange polymer attached to a reinforcing fabric by means of an interlayer of porous expanded PTFE. A coating of an ion exchange resin was present on at least a portion of the interior and exterior surfaces of the fabric and porous ePTFE.
U.S. Pat. No. 5,082,472 discloses a composite membrane of microporous film in laminar contact with a continuous ion exchange resin layer wherein both layers have similar area dimensions. Surfaces of internal nodes and fibrils of ePTFE may be coated with ion exchange resin coating. The ePTFE layer of this composite membrane imparts mechanical strength to the composite structure. U.S. Pat. Nos. 5,094,895 and 5,183,545 disclose a composite porous liquid-permeable article having multiple layers of porous ePTFE bonded together with interior and exterior surfaces coated with an ion exchange polymer.